1. Field of the Invention
The present invention relates to a flexible tube, and particularly to a flexible tube serving as a vibration-absorbing pipe connected to the exhaust system of an engine mainly for vehicles or the like to absorb the vibrations from the engine.
2. Description of the Related Art
The exhaust system for a vehicle is conventionally formed by connecting a catalytic converter, a sub muffler, a main muffler, and the like to an exhaust pipe extending from an exhaust manifold of an engine. To prevent the propagation of vibrations from the engine to the mufflers and the like situated at the rear stage of the exhaust system, it is known, for example, to provide a flexible tube serving as a vibration-absorbing pipe in the course of the exhaust pipe between the exhaust manifold and the catalytic converter so that the vibration from the engine can be absorbed.
For example, Japanese Patent Application Laid-open No. 7-42548 discloses a flexible tube including a cylindrical bellows tube, a cylindrical interlock tube placed inside the bellows tube, an upstream-side joint pipe and a downstream-side joint pipe coupled respectively to the two coupling-end portions located at the opposite ends of the bellows tube. The bellows tube includes a bellows portion located between the two coupling-end portions. In the bellows portion, ridge portions and valley portions are alternately formed in the axial direction of the bellows tube. The upstream-side joint pipe and the downstream-side joint pipe each include extension portions which extend from a portion thereof coupled to the bellows tube towards the axial center thereof, and the extension portions are fitted and fixed respectively to the inner peripheral surfaces of fixing end portions that are located at the respective opposite ends of the interlock tube.
In the flexible tube disclosed in the above-described Japanese Patent Application Laid-open No. 7-42548, the extension portions of the upstream-side and the downstream-side joint pipes are fitted to the inner peripheral surface of the interlock tube formed with a constant diameter that does not change all along the length of the interlock tube. Accordingly, a “coupling step” is formed between the interlock tube and the tip end of the extension portion of each of the upstream-side and the downstream-side joint pipes. This “coupling step” expands the exhaust-gas passage between the upstream-side joint pipe and the downstream-side joint pipe.
However, the expansion of the exhaust-gas passage by such “coupling step” fails to effectively reduce the exhaust noise or to reduce the back pressure (a pressure preventing the exhaust gas from being discharged). Further, the very existence of the “coupling step” requires the outer diameter of the bellows tube to be expanded, causing a problem that the flexible tube has to be larger in size. In addition, the exhaust gas flowing through the exhaust-gas passage has turbulences of flow at the “coupling steps,” which undesirably increases the flow resistance of the exhaust gas.